Resonant cavity filter



y 5, 1953 H. MAGNUSKI 2,637,782

RESONANT CAVITY FILTER Filed Nov. 28, 1947 2 SHEETS-SHEET 1 .19 J0 TO ANTENNA CAVITY OR ARrIiriNA WAVE SIGNAL r DEVICE CAWTY WAVE SIGNAL! 4 FlLTEE f DEVICE WAVE SIGNAL DEVICE WAVE SIGNAL To ANTENNA 20 DEVICE o2 ANTENNA A RELAY WAVE SIGNAL DEVICE 21- B TO ANTENNA RECEIVEK INVENTOR. fle zg/ 17 zzzasvfa' TRANS- A TRANS B TRANS C 5, I953 H. MAGNUSKI 2,637,782

RESONANT CAVITY FILTER Filed NOV. 28, 1947 2 SHEETS-SHEET 2 53 j" i J4 47 {y I g3 I 1| P l 6U 61? f 1f iii J61 s "a! I W M w 1 7 1! I JNVENTOR. H M fi jy yzz sza Patented May 5, 1953 RESONAN T "CAVITY FILTER Henry; Magnuski', v, Chicago; Ill;,-, assignorc-l to Motorola, Inc., Chicago, 111 la. corporation...

o f lllinois ApplicationNovember-28, 194-7; Serial N63788397i 3.: Claims:

'Ih'isdnventionrelates more particularlytoresonantcavityiilters adapt? ed to permit the operation ofa plnralityr oftwaveu signal devicesein the-zveryhig-hb frequency range.

from a single antenna;

In'tlie prior art various attempts; haver-b'een they can b tuned "to. any frequency the :hand" of frequenciesto be used: by -:the transmitters and: receivers with l which they: are associated. This. permits. filters of a standard: construction tot he" used in all cases resulting: in reduced" cost2asrtcomparedto as. system in whicli eachr filter: must the individually designed fora particularnfrequency;

For-filters: to he eifective to prevent interference:

between-wave r signal devicesaperatingr onvclosely adjacent channels with a single: antenna; the: filters? must be" capable" of 1 kieing ivery criticallf; tuned.- TlliS IEQllilGS: that" the filters must; not: change characteristics: with changes operation: conditions such as changes internperaturea It istherefore, anobject of th'e present invention torprovide an improved' filterz'which is highlyselective; which earn be: tuned through. a: range of frequencies and: in which the frequency-is :not changed "by'varying operating-"conditions:

A further object" 0f"i3hlS'? invention* is 15118?" pro.- visiorr of" an eiifcient system includihgmesonant cavity filters for connecting a plurality of wave signal devices to a single antenna in" which the filters prevent interaction between the: devices; attenuate spurious radiation" from the: transmit; ters, and decreas spurious responses in thereceiversz A feature ofthis inventionnisthe provision of a coupling system including: a- "pluralityot "wave signal devices operating::-orr: predeterrnined free quenciesiHahand :of 'frequenciesPinitheWerWhigli frequency range;v with 1 resonant: cavity filters which are?tunedxtolthmfrequency otithe device withtwhichypthey:are zassociat'edficouplingntiie deevicesto aacommonr'antenna.

ajlfurtherrfeaturezor?thisa:inyentionr:is;;tne:-;.pro: visiomoigaehiehlnselective --re scn.an -:,cevi-tn filter generally-*- to: filtersranda includinacouplin ggme ans ionconnectingfthe qfilter to naacoaxial' line, in such-imannerthat-relatively smallsinserticnyllosses-areproduced thereby.

Amsti'll furthemfeature oilthis. invention. is the provision of? a: resonant-cavity filter having; a-

movable: conductor-:ior tuning thesame andta support for the-movable conductor. which auto, matically compensates" for. changesin tempera-- ture-so-that{the frequencyrcfathe filter does not change: with changecin; temperature.-

Further: objects, features and advantages u will be apparent from .a consideration of the following descriptionitaken in. connection with the accom-- pan-ying drawings in which:

Fig. 1 illustrates a system for coupling. .three wave signal-devicestd a commonv antenna Fig zrillustrates a ,similar system usin larplurality -of filters in series;

Fig.1v 3 illustrates atsystemlin which a plurality of transmitters. may be; coupled, through cavity filters. to a}, relay which. selectively connects. the, transmiterslorl areceiver toan antenna;

Fligw l rillustrates lat-structural configuration of amresonant cavity. filter in accordance vwith. the invention;

Fig, {visa-top vieivloigthefilterl of Fig.4; and

Fig; .6 is aldetaill'view ot a coupling loop of the. filteruoflFig} 4;.

Inupracticing theinvention theresislprovided a system for connecting a.-.plura1ity. of wavesignal devicesgto a.-.- single. antenna. without causing .objectionahle interactionbetweenthe devices. v This. isacpomplishedjby connectin oneflor more-resanant' cavity filterswtuned tov the frequency of each device;. between. each devices and. the antenna. Each-filter; comprises: a cylindrical can-including. terminals-connectedmtocoupling loops to which the-signalis.v applied'and'from which it is received. Thefllterfis'in-effect a shorted resonant line and includes a" centerconductor having a movable portion which iseffective to determine the resonant-frequency *of-th'e cavity filter; Adjustable" supportingv means is I provided for the movableeportlorroffthe center 'conductoi' so that the filterrcambe tuned?tozanyirequency within az.lrangesof:frequencies. The supporting means is composed ofcimemberszthematerial of which is selectedvto have such temperature; characteristics thatlthe tuningotthe filter will notice disturbed low-changes in temperature.-

Referringynow morerparticularlv to the -draw-' ingsiin Figs-.1 12 to.-.=3 inclusive; there are illustrated: threeesystems in which. resonant cavity filterslin accordanceewith therinvention areused toipermittconnectionnf er-plurality of .wave signal devices to the same antenna. In Fig. 1 three wave signal devices, A, B and C are illustrated which may be three transmitters, three receivers, or any combination of transmitters and receivers. The wave signal device A indicated by reference character In is connected by coaxial cable II to resonant cavity filter I2, the wave signal device I3 is connected by coaxial cable l4 to resonant cavity filter I5, and the wave signal device I6 is connected by coaxial cable I1 to the filter 8. The resonant cavity filters l2, I and I8 are tuned to be resonant at the frequency of operation of the wave signal devices I0, I3 and I6, respectively, so that signals of the particularfrequency to which the filters are tuned are passed thereby without substantial attenuation whereas other frequencies are greatly attenuated by the filters; It is obvious, therefore, that the filter |2 will provide a conducting path between the coaxial cable 'I 9 connected to an antenna and the device H! for signals of the frequency at which --the device It! is tuned to operate. The filters 5 and 8, however, will prevent signals of this frequency from reach ing the devices I3 and I6 so that operation of the device ||l will have substantially no effect on these devices. Similarly, the filter l5 will pass signals of the frequency of the device l3, providing a conducting circuit between the device I3 and the antenna. These signals will be prevented from affecting the devices I0 and I6 by the filters l2 and I8 which will attenuate the signals of the frequency of the device l3. It is obvious that more than three devices may be operated with the same antenna by providing an equal number of filters.

The system in Fig. 2 is similar to the system in Fig. 1 except that the wave signal devices 2|] and 2| are each connected to the antenna by two resonant cavity filters connected in series. Filters 22 and 23 are connected between the wave signal device 2!] and the antenna, and filters 24 and 25 are connected between device 2| and the antenna. The filters will operate in the manner described in connection with Fig. 1 to provide a conducting path between the associated wave signal device and the antenna only for signals of the frequency to which each cavity filter is resonant. By using the filters in series added selectivity is obtained. It is obvious that a larger number of filters can be connected in series to obtain greater selectivity, the number being limited only by the losses in the filters as will be explained.

In Fig. 3 there is illustrated a system in which three transmitters 30, 3| and 32 and a receiver 33 are all adapted to be operated from a single antenna. The transmitters are connected through resonant cavity filters 34', 35 and '36 to relay "3! which is arranged to connect either the three transmitters or the receiver to the antenna. This is accomplished by movable contact 38 which is actuated by coil 39 which may be energized from a remote source to thereby provide remote control of the relay 31. The contact 38 is illustrated in the position in which the transmitters are connected to the antenna with the three transmitters being capable of simultaneous operation due to the isolating effects of the resonant cavity filters 34, 35 and 36. These filters function in the manner previously described in connection with Figs. 1 and 2 to provide a conducting path only for a particular frequency which in each instance will be the frequency of the transmitter to which it is connected. It is apparent that the contact 38 maybe moved to the right so that the 4 receiver 33 is connected to the antenna and the transmitters are disconnected.

Reference is now made to Figs. 4, 5 and 6 which illustrate the physical construction of the resonant cavity filter in accordance with the invention. In Fig. 4 the cavity is illustrated as including an outer conductor in the form of a cylindrical can 40 having a bottom closure 4| and a top closure 42 with a central opening 43 therein. Secured in the opening 43 is a tubular member 44 which extends concentrically into the can 46 a substantial portion of the distance to the bottom closure 4|. Inside the tubular member 44 is a plunger 45 which combines with the tubular member 44- to provide a center conductor of adjustable length. The can 40 and center conductor thereby form a resonant line which is shorted at the top by closure 42. The plunger 45 is supported by an actuating rod 46 which is adjustably mounted on bracket structure 41. The bracket 47 issupported on an annular top closure 48 which is secured to the can 40 at its outer edge and having the inner diameter thereof positioned closely about the tubular member 44. A pairof nuts 49 and 50 threaded on rod 46 are positioned at either side of the bracket 41 to permit the rod 46 and accordingly the plunger 45 to be adjustably positioned with respect to the can 43. As is apparent from Figs. 4 and 5, the bracket 4! includes upstanding portions 5| which are secured to the top member 48. For making the portions 5| more rigid, bracing members 52 may be provided. A bridge member 53 connects the two upstanding portions 5| and includes an opening 54 which receives the rod 46. As previously stated, theplunger 45 is movable within the tubular member 44 and includes a portion 55 which extends beyond the tubular member. The tubular member 44 includes a reduced end 56 which engages the plunger 45 in a sliding fit to support the plunger and make electrical contact thereto.- In the very high frequency range, the cavity filter may be used as a quarter wave resonant line, and by changing the position of the plunger 45 by adjustment of the nuts 49 and 50, the length of the line can be changed so that the filter can be tuned to be resonant at any desired frequency within a predetermined range. For use in a system operating in the frequency range from 152 to 162 megacycles, the over-all size of the filter is slightly more than two feet. I The tubular member 44 and the plunger 45 of the center conductor also form a resonant line the length of which is substantially equal to a quarter. wave length of the frequencies to which the filter is tuned.- As these two.members are spaced from each other at the top to form an open line, they will in effect be shortedat-the bottom. Therefore, the filter will operate satis-. factorily even though good electrical contact is not made between the end 56 of the member 44 and the plunger 45 due to the shorting effect of the open line.

Changes in length of the center conductor due to changes in temperature are accomplished by design of the actuating rod 46 and the bracket 41. The rod 46 is anchored to the plunger 45 by connection to the insulating spacer 51 which is in turn connected to the plunger 45. The spacer 58 at the top of the plunger has an opening of such size that the rod 46 can slide therein. By making the rod 46 and the bracket 41 of materials having different coeflicients of'expansion, and by properly dimensioningthe .rod and ans'ri'zaai pensateefonz hangesiimidimensions oitthe: filter-i due to-:,te1ma,eraturecchtanges'v toztherebytholdethe- IBSODZEE, frequencye'ofe' tliecfilterv constant; In actualffipractice 1 it i-has 3been; founcksatisfaotoryato. makBlithBIfEbTaCkEt-i of coppen-and 'i the :1 adjusting rodrofznickel steel}havingiaa veryrsmall coenicient oft expansions By desighinge the: components the-mroportion"aillustratedgimthe:drawing; automatics temperature: compensation base been. achieved.

The+- resonant.cavity-miter; includes. a; pain of. coupling loopssfifl and whiche form; @th'e::input1 and .output 1 COHHEGtlOIlSqtO thee filter ThQrlOOpEi; areeppsitioned at the-shorted endqof ithe resonant: line; .so that the -willsbeapositioned the most;- mtensee-fifleld. As'eal-lustratedeir Fig fijatherloops are made' offlat'stripsrof'metal hich are doubled-.- back ,to'g-form asingle.-Llrshaped:-turrr.v By; using:- flatistifipsv having relatively-large area,.the -capac-'--. itytof {6116:1009@flillllitlengthyis increased. This: isgdesirableiso; that; theicharacteristicn impedanca of, the: loop will, .begapproximately equal" .to that; ofzthe .coaxial line ,to which: itii s. connected, for the frequencies being'used, as when .this .con dition -exists. maxim-umselectivity is obtained. That'is, the U-shaped't'urn is in effect a transmission line and, 01f resonance tends to form a short circuit across the-..cable connected thereto. As the filterisperfectly'syrrmretrical; it is immaterialfas to which of thenoop's is used foi' input or." output: Each. of. .thetl'oopsiris connected to a flttingi6=2 adapted tolreceive axcoaxial cable; the fittings being. arranged. so. that' 'the. 1001151 5 Q :areficonnemed betweenitmt'centel' Conn-5cm? J ceive'r'in any" location will'be at thesame distance andtthershield of the coaxialncablei Thefittings 62;a'lso connect the. shields ofsth'e cableto the can dn wh-ich may be at ground potential:

As previously. stated,.,the. *fiiter= is tuned-"by adjusting the positiorrof: thew-plunger id-with respectftox-the can 40. The filter-is particularly applicable for use in the very high frequency range as for example from 100 to 390 megacycles, but it is apparent that by properly dimensioning the components, the filter can be designed for use in other frequency ranges. 4

Sufficient tuning range may be provided in the filter to correspond to the tuning range of standard transmitters and/or receivers. For example, filters have been constructed which are adjus able through the range from 152 to 162 megacycles. The filter has a very high Q so that for the frequency to which the filter is tuned the attenuation produced thereby will be very small. However, when frequencies other than the frequency to which the cavity is tuned are applied thereto, the cavity becomes substantially a short circuit, and by making the connections to the cavity an odd multiple of a quarter of a wave length the signals of other frequencies are substantially eliminated. The resonant cavity itself presents a high impedance when frequencies other than the frequency to which it is resonant are applied thereto. This results in small \currents in the cavity and through the transformer action of the input coupling loop, the voltage across the loop is small to effectively form a short circuit.

For maximum selectivity, the loops must be relatively small. However, this results in minimum loading of the filter and the insertion loss of the filter, therefore, becomes a maximum. If it is not necessary to have such high selectivity, it is possible, by the use of large couplin loops having large surface area, to obtain smaller insertion loss at the sacrifice of high selectivity.

simple: as possible.

Ita is}, therefore... seen that ,the ,characteristics.=;of1 the couplingiloopswmay,beselected -to;;-fit apa ticular situation and. permit the operation oftthe filter with small insertiongloss when high selec. tivi-ty is -notmecessary As: stated, above, the .,filters-. can. be-sused to eliminate. spurious; responses in a. receiver. The filter is" particularly, advantageous. for this i use: asvthe selectivityisprovided- .at' the. antenna. endiof. the; receiver so that-undesirable frequenciesi 1, then filter is also particularly advantageous,.for

use in relay stations wherein signals are-.retrans i mitted on,different frequencies. v In such applica. tions, it is :highly; desirable thattheequipment be I held toa; minimum. and the installationibepas These, requirements. areemet: inthe system: disclosed-.1-

The .systemsiusin the *filter in .accordancewithg theinvention to operate a: plurality of; transmits: ters on the same; antenna provide more practical.

; systems than theuse ofseparate-antennas spaced:

from each other in many instances. liorexam pie, when. it desiredtoreceive signals :from one stationwheninthe close vicinityof a second station,the secondstation may maskout the desired signals from: the first station. This mayhappem even if the signals are of. substantially, equalil strength., This difficulty is. completely'avoided when? the two transmitters are. positioned. at. the same location and use. the samev antenna as a re,-

from both transmitters and normal selectivity will" permit the reception of fwhichever. signal is. desired.

While? there have, been describedj certain .embo'dirn'ents of f my invention which are .illustrative.= thereof; it is apparent. thatvarious changes. and} modifications 'canbe made therein without departing from the intended scope of the invention as defined in the appended claims.

I claim:

1. A resonant cavity filter tunable through a range of frequencies comprising a cylindrical can having closures on either end thereof, one of said closures having a circular opening in the center thereof, a tubular member extending in said can having a first end positioned in said opening in said one closure, and a second end spaced from said other closure, a plunger positioned in said tubular member and extending beyond said memher at said second end thereof, an actuating rod secured to said plunger at one end thereof, and a bracket secured to said one closure for adjustably supporting said rod, said tubular member having a reduced portion at said second end engaging said plunger for making electrical contact thereto and for supporting said end of said plunger which extends beyond said member, said tubular member and said plunger bein of such length to form a quarter-wave resonant line which is open at said first end of said tubular member and effectively forms a short circuit between said second end of said tubular member and said plunger at frequencies within said range, said rod and said bracket being made of materials having different temperature coefiicients of expansion and being of such dimensions that the position of said plunger is automatically adjusted to hold the frequency of said filter substantially constant through variations in temperature.

2. A resonant cavity filter adapted to be inserted in a coaxial line for selecting signals of a particular frequency transmitted along said line, said filter including in combination, a cylindrical conducting can having conducting closures at the ends thereof, one of said closures having an opening in the center thereof, a central conductor structure extending in said can having a first portion secured in said opening in said one closure and a second end portion spaced from said other closure, and coupling means secured to said can on opposite sides thereof adjacent said one closure, said coupling means including an outer cylindrical conductor connected to said can and an inner conductor insulated from said can for connection to the conductors of the coaxial line, each of said coupling means including a U-shaped coupling loop connected between said outer and inner conductors and extending into said can to- Ward said central conductor structure, said loops being made of fiat conducting material with the fiat arms thereof extending parallel to each other and being relatively closely spaced so that a relatively large capacity is provided thereby, said loops being constructed to have impedances substantially equal to that of said coaxial line within the range of frequencies of the filter.

3. A resonant cavity filter adapted to be inserted in a coaxial line and. tunable through a range of frequencies for selecting signals of a particular frequency within said range transmitted along said line, said filter including in combination, a cylindrical conducting can having conducting closures at the ends thereof, one of said closures having an opening in the center thereof, a tubular member extending in said can having a first end secured in said opening in said one closure and a second end spaced from said other closure, a plunger positioned in said tubular member and having a portion extending beyond said second end thereof, said tubular member including an inturned portion at said second end for mechanically supporting said plunger and making electrical connection thereto, adjustable supporting means for said plunger secured to said one closure and movable to change the length of the portion of said plunger extending beyond said second end of said tubular member to thereby change the resonant frequency of said cavity filter, and coupling means secured to said can on opposite sides thereof adjacent said one closure, said coupling means including an outer cylindrical conductor electrically connected to said can and an inner conductor insulated from said can for connection to the conductors of the coaxial line, each of said coupling means including a U-shaped coupling loop connected between said outer and inner conductors thereof and extending into said can toward said tubular member, said loops being made of flat conducting material with the arms of the U extending parallel to each other and being relatively closely spaced so that a relatively large capacity is provided thereby, said loops being constructed to have impedances substantially equal to that of said coaxial line within the range of frequencies of said filter.

HENRY MAGNUSKI.

References Cited in the file of this patent UNITED STATES PATENTS 

